1. Field of the Invention
The present invention relates generally to servo control in tape drives. In particular, the present invention provides control methods and apparatus for use with magnetic tape having one or more servo tracks providing transverse position information during read/write operations.
2. Description of Related Art
In recent years, the capacity and performance of tape storage systems has increased considerably, and the potential for further growth appears to be substantial. In order to achieve higher cartridge capacities and improved performance, advances in several technical areas are necessary. Areal density increase, i.e. increase in linear and/or track density, is key to achieving higher storage capacities. In the former case, the distance between adjacent bit cells decreases, leading to an increase in interference between characters. Higher track density implies narrower track width, narrower read/write heads and closer head spacing, leading to losses in signal-to-noise ratio. Also, issues of intertrack-interference become critical.
Reliable servomechanisms for controlling the tape transport and tape/head positioning systems, such as reel-to-reel and track-following servomechanisms, are therefore important for achieving best overall drive performance.
Tape storage systems typically use dedicated servo tracks recorded alongside the data tracks on the tape to provide positional information for use by the servo control system. In linear tape open (LTO) tape drive systems for example, the timing-based servo (TBS) format has been adopted as a standard. The TBS system defines a format for the servo pattern recorded in the servo tracks and is detailed in Standard ECMA-319, “Data interchange on 12.7 mm 384-track magnetic tape cartridges—Ultrium-1 format,” June 2001, pp. 48 to 56. The TBS servo pattern, described in more detail below, has a frame format wherein magnetic transitions define a series of stripes with two different azimuthal slopes. During read/write operations, the transverse position of the read/write head can be derived from the relative timing of pulses generated by a narrow servo reader head reading the stripe pattern. TBS patterns also allow the encoding of additional longitudinal position information without affecting the generation of the transverse position error signal (PES). This is obtained by shifting transitions (stripes) in the servo pattern from their nominal pattern position in the longitudinal direction of the tape.
In accordance with the LTO standard, servo tracks are recorded in servo bands which extend on either side of each of several data bands spaced transversely across the tape. Different servo readers can be arranged for reading servo tracks in different bands concurrently during read/write operations, thus providing more information to the servo control system. For example, in current IBM tape systems there are four servo readers arranged on two modules of the read/write head as illustrated in FIG. 1 of the accompanying drawings. Each of the left and right head modules shown schematically here has two servo readers S, one above and one below a line of alternating read and write elements, labeled R and W respectively, only partially shown in the figure.
The two servo readers S on each module are thus arranged for reading respective servo tracks on either side of the current data band. This modular head arrangement allows reading and writing in both directions of tape travel, with writing always being performed by the leading module. As alternate tracks are written by the write elements W of the leading module, the two servo readers of that module are active, and the read elements of the trailing module are used to verify the written data.
For a read operation in a given direction of tape travel, the two servo readers of the module with active read elements are operative. Thus, two servo readers on one of the two modules are active at all times during read/write operations. A transverse position-error signal (PES) can be derived from the servo read signal from each of the two active readers S. In particular, an estimate of the transverse position of a given reader S (relative to a desired or nominal position, typically the servo track centre line) can be derived from the servo read signal for each frame of the TBS servo pattern. Reading of successive servo frames by reader S results in a series of transverse position estimates for time instants corresponding to reading of respective servo frames. The resulting position-error signal is used together with tape speed and LPOS information derived from the servo pattern for track-following and reel-to-reel servo control functions.
Among the main problems adversely affecting the performance of track following and reel-to-reel servomechanisms, as well as the quality of readback signals in data channels, are the dynamic skew and the variation of tape tension. Dynamic skew arises when the head does not remain perfectly perpendicular relative to the direction in which the tape moves. Tape skewing with respect to the head during tape motion tends to produce increasing loss of signal as frequency rises as well as readback-signal frequency fluctuations. These cannot easily be tracked by the phase-lock loops that are usually implemented in detection systems. Moreover, tape skewing introduces delays in the data channels that have to be estimated and compensated for. When operating in the steady state velocity mode, variations of tape tension around the nominal value, also called once-arounds, are induced by the reel eccentricities. In tape transport, this problem is particularly serious when the reel rotation frequencies are near the resonance frequency determined by the tape path. Tape tension errors affect the position error signal and hence the performance of track following servo.
The adverse effects of dynamic skew and tension variation are well known and understood, and various approaches have been proposed to estimate these quantities to improve drive operation. For example, U.S. Pat. No. 6,563,659 discloses a system for estimating changes in tape tension based on detected changes in distance between two transversely spaced servo bands. The changes in distance are determined from position-error signals derived for two servo read elements reading respective servo bands.
A system described in U.S. Pat. No. 6,934,108 measures changes in tension based on changes in longitudinal tape length, particularly changes in longitudinal distance between two known points in the servo pattern read by a servo read head. Tape reel motor torque is then adjusted accordingly.
U.S. Pat. No. 4,062,047 describes test equipment for separately measuring read head skew and write head skew in a tape recording system. Read head skew is determined by reading a reference tape, and write head skew is determined by reading a pattern written by the write head using a properly-aligned read head. In both cases, the skew estimates are based on time differences between corresponding peaks of identical timing signals recorded in two transversely spaced tracks read by respective read heads.
U.S. Pat. No. 6,430,008 senses tape skew based on timing differences between signals from two transversely-spaced servo read elements reading respective servo tracks, with tape head positioning being controlled accordingly.
U.S. Pat. No. 6,724,559 discloses a similar system in which the detected skew is corrected either by rotating the tape head or by tilting rollers in the tape path. While these techniques make some manner of provision for one or other of tape skew and tension variation, an improved system would be highly desirable for the reasons discussed earlier.